I had the same problem turning over mids for a while. I'm not sure but I think the problem was off-axis torque. To stop the OAT, i worked on getting the follow-through right: with wrist rotated to keep the thumb down, and with my arm finishing at shoulder height, not lower.
Nowadays, I don't actively think about those things any more, and all of my mids behave WAY more overstable of the way they used to. Like, what used to be my understable midrange, a beat DX roc, is now my overstable midrange, and it only turns over at 100% power.
Champion Teebird, Star TL, Buzzz, Wizard, Aviar